Many studies, in bacteria as well as in other microbial organisms, have shown that some repair deficient strains are more mutable than the wild types. Studies with ad-3 test system of Neurospora crassa have shown that repair-deficient strains, upr-1 and uvs-2, are more mutable than wild type after UV or gamma ray irradiation. It has also been shown that the spectra of genetic alterations caused by UV in repair-deficient strains are different from that caused by UV in the wild type. It is of interest to study and compare the mutagenic sensitivity of repair-deficient and wild type strains to various chemical carcinogens and to compare the spectra of genetic alterations caused by these carcinogenic agents in the UV-sensitive and wild strains.